The coupling between monsoon rainfall and Sea Surface Temperature in the subtropical South Atlantic during the Last Glacial Period

The long-term variations in rainfall amount over southern Brazil are thought to be mainly driven by insolation forcing on the South American Monsoon System (SAMS). However, the bimodal rainfall regime over the region limits the interpretations from pioneering studies using δ¹⁸O in stalagmites. Here, we display high-resolution trace elements data from the Botuverá Cave (27°S) as a proxy for paleo-precipitation amount covering the Last Glacial Period (LGP). Our record highlights an in-phase variation between Mg/Ca, Sr/Ca, and δ¹⁸O for the first two insolation cycles, suggesting increased precipitation due to enhanced monsoon activity during high austral summer insolation. From 60 to 10 ky BP, our geochemistry data do not follow insolation equally well, suggesting that Atlantic Meridional Overturning Circulation (AMOC) might interplay with precession as drivers for long-term paleo-precipitation changes in southern Brazil. We compare the speleothem record to marine SST records to identify feedback mechanisms between the atmospheric circulation and South Atlantic SST characterized by a stronger SAMS and colder SSTs relationship in adjacent South Atlantic. Two modes of precipitation variability are proposed: 1) from 113.5 to 40 ky BP, an in-phase precipitation variability between the areas affected by the South Atlantic Convergence Zone (SACZ) and southeast South America (SESA); and 2) from 40 to 10 ky BP, a precipitation dipole is identified between SACZ and SESA, respectively. For the millennial timescale, our Sr/Ca and Mg/Ca data indicate that there exists a relationship between higher precipitation and enhanced SAMS during some Heinrich Stadial (HSs) and Glacial Interstadial (GI) events. We argue that the monsoon circulation and its related cloudiness band play an important role in affecting SST changes over the South Atlantic at distinct timescales.